Optical fibers are in widespread use today as transmission media because of their large bandwidth capabilities and small size. However, they are mechanically fragile, exhibiting low strain fracture under tensile loading and degraded light transmission when bent. Accordingly, cable structures have been developed to protect mechanically the optical fibers thereby rendering them a realizeable transmission medium.
An optical fiber cable must be capable of withstanding tensile loads applied when the cable is pulled into ducts, for example, and bending stresses caused when the cable is pulled through turns in the ducts and bent when being introduced through manholes. An optical fiber cable suitable for such use is disclosed in R. A. Kempf et al U.S. Pat. No. 4,078,853 which issued on Mar. 14, 1978. In one embodiment, a core of optical fiber ribbons is surrounded by a plastic, loose-fitting inner jacket, a compliant layer of plastic twine and a plastic outer jacket which is reinforced with primary strength members that are wrapped about the twine. The strength members are embedded in the outer jacket to achieve substantial coupling therewith.
In some situations, increased tensile loads may be encountered, such as, for example, where ducts are extremely conjested or where the ducts have more bends than usual. If more strength members are added to the above-described cable to meet these increased requirements, bending flexibility which is so necessary to ease cable handling and installation decreases. A cable which is capable of resisting relatively high tensile loads while exhibiting bending flexibility is disclosed in U.S. Pat. No. 4,241,979 which issued on Dec. 30, 1980 in the names of P. F. Gagen and M. R. Santana. In it, the coupling between the strength members and an outer plastic jacket is precisely controlled. A bedding layer of material, about which the strength members are wrapped helically, is added between a plastic inner jacket and the outer jacket to control the extent to which the strength members are encapsulated by the outer jacket. By preventing encapsulation of portions of the strength members, the strength members are still tightly coupled to the outer jacket under a tensile load, but are capable of sliding with respect to the outer jacket under local bending where no encapsulation occurs. Under tensile loading, sliding is eliminated substantially because sufficient shear and frictional coupling exist between the outer jacket and the strength members. In one embodiment, two reinforcement strength member layers are wrapped helically in opposite directions. Under a tensile load, these two layers of strength members produce equal but oppositely directed torques about the longitudinal axis is the cable to ensure the absence of torsional creep under sustained tensile loads.
Optical fiber cables also may be strung between poles or buried in the ground thus exposing them to abuse such as, for example, attack by rodents, mechanical abrasion and crushing. Rodents have been able to encompass the cable with their teeth and pull open the seam of a steel shield intended for rodent protection. Moisture which enters the cable through rodent-caused openings in the jacket causes the common grade steel shield to corrode.
Both buried and aerial cables also are damaged by lightning strikes. Thermal damage, that is burning, charring and melting of the sheath components, is caused by the heating effects of the lightning arc and a current being carried to ground by the metallic members of the core or sheath. A second mode of lightning damage is mechanical, causing crushing and distortion of the sheath. This results from an explosive impact, sometimes called a steamhammer effect, which is caused by the instantaneous vaporization of water in the earth in a lightning channel to the cable.
A cable which provides suitable protection against rodents and lightning is disclosed in U.S. Pat. No. 4,557,560 which issued on Dec. 10, 1985 in the names of W. D. Bohannon Jr. et al. In it, a core is enclosed in a shield made of a highly conductive material such as copper, for example, and in a corrugated outer shield comprising a corrosion-resistant metallic material such as stainless steel to which is bonded an outer jacket. Even if the outer jacket is violated by rodents and the outer shield is exposed, the stainless steel does not corrode and the integrity of the inner portions of the cable is preserved. Also, the bonding of the jacket to the outer shield helps to prevent lifting of the seam by rodents.
As should be apparent, this last design cable and others which are commercially available have added lightning and rodent sheath protection to an existing cable design. This has resulted in a cable which may be unnecessarily large in outer diameter and one which may require excessive manufacturing floor space, material and labor. In Appl. Ser. No. 825,291 which was filed on Jan. 31, 1986 in the names of W. D. Bohannon, Jr. et al, an optical fiber core is enclosed in a sheath system which includes a single metallic shield enclosed by a plastic jacket. A plurality of individual longitudinally extending strength members are caused to be disposed in engagement with the shield. This last-described arrangement results in a cable having a significantly smaller diameter than prior art cables. However, the manufacture of a cable in which wire-like strength members are wrapped about an advancing core requires the use of a relatively low line speed.
What is needed and what seemingly is not provided by the prior art is an optical fiber cable having a strength system which may be manufactured simply and inexpensively with existing equipment. The sought-after strength system should be one which easily is integrated with other elements of a cable and with different shield systems depending on the anticipated use of the cable.